Preterm birth is the leading cause of death in the first month of life and a contributing cause in more than a third of all infant deaths. Intra-amniotic infection (IAI) is one of the leading causes of idiopathic preterm birth <37 weeks of gestation. Other conditions associated with preterm birth include preterm labor, preterm rupture of membranes, preeclampsia, abrupta placenta, placenta previa, fetal growth retardation, excessive or inadequate amniotic fluid volume, fetal anomalies, intrauterine hemorrhage, diabetes, drug abuse and stress. Management of preterm labor and preterm birth may include treatment with tocolytic agents, and corticosteroids for fetal pulmonary maturation, if indicated. Narrow-spectrum antibiotics may be prescribed for Group B Streptococcus coverage pending negative culture results.
IAI is one of the most important causes of idiopathic preterm labor and preterm birth. IAI is a microbial invasion of the amniotic cavity and occurs in 10-15% of all preterm labor cases. (Newton E R. Clin Obstet Gynecol 1993; 36(4):795-808; Watts D H, et al., Obstet Gynecol 1992; 79:351-7; Romero R, et al., Am J Obstet Gynecol 1993; 169:805-16; Hillier S L, et al., Obstet Gynecol 1993; 81:941-8). Other terms used to describe IAI with or without intact membranes include: amniotic fluid infection, amnionitis, and clinical chorioamnionitis. In addition to the role of IAI as a cause of preterm labor, IAI is also associated with increased neonatal morbidity and mortality, particularly among preterm neonates. In general, a three to four-fold increase in perinatal mortality has been observed among low birth weight neonates born to mothers with IAI. There are also increases in respiratory distress syndrome, intraventricular hemorrhage, and neonatal sepsis. (Morales, W. J. Obstetrics and Gynecology 70:183, 1987). IAI has been independently implicated in neonatal periventricular leukomalacia and cerebral palsy; the risks of cerebral white matter damage and cerebral palsy are nine-fold greater in the setting of IAI. (Bejar, R., et al., Am. J. Obstet. Gynecol. 159:357, 1988; Grether, J. K. and Nelson, K. B. JAMA 278:207, 1997).
The majority of IAI cases, 80% to 90%, are subclinical (asymptomatic) other than preterm labor. Currently, the management of idiopathic preterm labor includes observation, treatment with tocolytic agents and possible confirmation of IAI by amniocentesis and culture. Amniotic fluid culture alone underestimates the true prevalence of IAI because of the presence of uncultivable microorganisms, difficulty in isolating fastidious microorganisms and previous antibiotic therapy (Romero, R. et al., Am. J. Obstet. Gynecol. 161:817, 1989). A positive IAI test or the present invention would provide a useful adjunct to the current diagnosis and treatment regimen available to the clinician. The accurate diagnosis of IAI is important for appropriate treatment of the mother with targeted antibiotics, withholding tocolytic therapy which is counterindicated in IAI as well as anticipating the location of delivery for the mother and the necessary level of care for the infant who may be very preterm and ill as an excess consequence of IAI.
A negative IAI test of the present invention provides reassurance that the etiology of preterm labor may be from sources other than infection. A negative test, in conjunction with 30 observation of other signs and/or symptoms, allows the physician to treat preterm labor.
Pathogenesis and Risk Factors:
Intra-amniotic infection likely occurs as a result of an ascending infection by lower genital tract microorganisms. The prevalence of IAI is strongly inversely associated with gestational age. (Watts D H, et al., Obstet Gynecol 1992; 79:351-7). Bacteria indigenous to the lower genital tract are recovered from the amniotic fluid of 10-20% of all women in preterm labor with intact amniotic membranes without clinical signs of IAI (Romero R, et al., Ann N Y Acad Sci 1991; 622:355-75) and in up to 67% of women in preterm labor with pregnancies ending at 23-24 weeks gestation. (Watts D H, et al., Obstet Gynecol 1992; 79:351-7). Moreover, these observations are supported by histologic chorioamnionitis which has been found in 60-90% of gestations ending between 20 and 24 weeks. These observations support the hypothesis that IAI is an important cause of idiopathic preterm labor, especially at early gestational ages.
Diagnosis:
An early diagnosis of IAI could allow timely treatment and intervention. However, there are multiple challenges in making the correct diagnosis. From the clinical perspective, early diagnosis is problematic because the clinical signs and symptoms of IAI occur late in the course of the infection, and are general and non-specific. The clinical criteria commonly used to diagnose IAI include preterm labor with maternal fever (≧37.8° C.), along with two or more of the following: maternal leukocytosis (≧15,000/mm3), maternal or fetal tachycardia, uterine tenderness, or foul-smelling amniotic fluid. (Gibbs R S, et al., Am J Obstet Gynecol 1980; 136(6):709-13). In a study by Watts, et al., of women with preterm labor, there was no difference in mean maximum maternal temperature, WBC count and differential between women with or without positive amniotic fluid cultures. Subclinical IAI is a term used to describe IAI and in which signs and symptoms are minimal or absent in approximately 88% cases with positive amniotic fluid cultures. (Watts D H, et al., Obstet Gynecol 1992; 79:351-7). The concept of subclinical IAI is further corroborated by the findings of Gravett, et al., utilizing a non-human primate model. These investigators demonstrated that following experimental IAI induced with Group B streptococcus, fever and leukocytosis are present only 50% of the time at the onset of infection-induced preterm labor, which occurs 28 to 40 hours after experimental infection. (Gravett M G, et al., Am J Obstet Gynecol 1994; 171(6):1660-7).
Because of the inconsistency of clinical features, other adjunctive laboratory tests have 30 been utilized to aid in the diagnosis of IAI. These include: measurement of maternal C-reactive protein, direct examination of amniotic fluid for leukocytes or bacteria on Gram stain, amniotic fluid culture, measurement of amniotic fluid glucose concentrations, detection of amniotic fluid leukocyte esterase, detection of bacterial organic acids by gas-liquid chromatography, measurements of various amniotic fluid cytokines (e.g., interleukins 2, 4, 6, granulocyte colony-stimulating factor, and tumor necrosis factor-.alpha.), matrix metalloproteinase-9, lactoferrin, and assessment of fetal activity (biophysical profile) by ultrasonography. Measurement of cytokines or other biochemical factors is expensive, generally not clinically available, and is primarily a research tool. Further, the testing efficiency of these tests has not been consistently better than more readily available traditional tests such as amniotic fluid Gram stain and culture, amniotic fluid glucose concentrations, and detection of amniotic fluid leukocyte esterase. The efficiency of these tests has been previously extensively reviewed. (Ohlsson, A. and Wang, E.: An analysis of antenatal tests to detect infection at preterm rupture of the membranes. American Journal of Obstetrics and Gynecology 162:809, 1990). Although all have reasonable sensitivity, specificity, and predictive value, none are sufficiently sensitive or specific to be utilized independently of clinical features in the diagnosis of IAI.
Accordingly, there is a great need for new approaches that allow early and accurate diagnosis of IAI.